The proposed work is to study the effects of selenium (Na2SeO3) on 1,2-dimethylhydrazine induced carcinogenesis. In an earlier study (Cancer 40:2557, 1977) the colon tumor incidence in DMH-treated male Sprague-Dawley rats was reduced from 87% to 40% by addition of 4 ppm Se to the drinking water. In the proposed study male SD rats will be provided 4 ppm SE supplements to the drinking water before, during and after DMH administration (20 mg/kg body wt. for 20 weeks). The goals are to determine whether: 1) Se delays the onset of tumors, 2) early stages of colon cancer induction can be inhibited by Se, and 3) colon cancer induction can be reversed by Se. The role(s) of Se in colon tumor induction and/or reversal will be elevated with histopathological, cytogenetic (SCE), enzymatic (eg. glutathione peroxodase, beta-glucuronidase, alkaline phosphatase) and other biochemical markers (e.g., Se and mucin levels). The acute toxicity of Se as Na2SeO3 in weanling and adult rats as well as the chronic toxicity of Se in male Sprague-Dawley rats will be characterized prior to initiation of the DMH-induced carcinogenesis studies.